Add base configuration

- Follow XDG standard: ~/.config and ~/.local folders
- Set environment variables and define aliases within ~/.config/shell
- Add installation script for easy setup
- Add README.md with info on the installation and general notes

README.md

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+# Dotfiles
+
+This repository contains useful (config) files.
+
+It is structured into two branches:
+- [desktop](https://code.webartifex.biz/alexander/dotfiles/src/branch/desktop)
+- [main](https://code.webartifex.biz/alexander/dotfiles/src/branch/main)
+
+`main` contains dotfiles intended to be used on all kinds of machines
+    and can be thought of as a "minimal" or "server" version.
+`desktop` is (re-)based on top of `main`
+    and adds "desktop" related dotfiles (e.g., GNOME stuff).
+
+
+## Installation
+
+Simply run:
+
+```sh
+curl https://code.webartifex.biz/alexander/dotfiles/raw/branch/main/.local/bin/install-dotfiles > install-dotfiles && . ./install-dotfiles && rm ./install-dotfiles
+```
+
+or
+
+```
+wget https://code.webartifex.biz/alexander/dotfiles/raw/branch/main/.local/bin/install-dotfiles -O install-dotfiles && . ./install-dotfiles && rm ./install-dotfiles
+```
+
+This downloads a simple [installation script](.local/bin/install-dotfiles)
+    and then executes it.
+The script has only one dependency, namely [git](https://git-scm.com).
+So, it should not be too hard to get this going.
+
+Normally, I advice against executing shell scripts from the internet,
+    but this one is short enough to be read even by beginners.
+So, convince yourself that it is not harmful!
+
+
+## Shells
+
+The config files in this repository are optimized for usage with
+    [GNU's Bourne again shell](https://man7.org/linux/man-pages/man1/bash.1.html),
+    or `bash` for short,
+    and the popular [zsh](https://www.zsh.org/).